PowerPoint Presentation

Optometry Tomorrow 2016

The Changing Face of TonometryJason Higginbotham BSc (Hons) MCOptom FBDO Ophthalmic Clinical Advisor Birmingham Optical

At present, under GOC and GOS requirements, Optometrists have to carry out assessment of the Intra Ocular Pressure (IOP) of patients during an eye examination. IOP is still currently used as a diagnostic marker for Glaucoma and of course Ocular Hypertension. There have been numerous papers and studies on the relevance of IOP in Glaucoma diagnosis and monitoring as well as its treatment. Instead, how IOP is measured and how accurate and reliable those measurements can be is the main topic of this presentation.1

The History of TonometryVisual loss related to a hard or marble like eye was first reported as far back as the 10th Century in Arabia (Al-Tabari).In 1622, Bannister first suggested Digital Palpation as a technique for checking the pressure of the eyeball. In 1862, Von Graefe invented the first indentation Tonometer.

Over the centuries, people have been aware of blindness that seems to have been caused by hard / glassy eyes. The term Glaucosis was originally used to describe a range of conditions causing blindness, without any real understanding of mechanisms. As far back as 800BC, people have referred to the juices of the eye causing the pupils to turn grey. This is most likely a reference to cataracts, but several physicians indepentently also noted that the use of deadly nightshade (Atropine) would cause the grey pupils to turn black. In 1854, Eduard Jaeger first described the optic disc appearance in Glaucoma in relation to some form of raised pressure in the eye. Von Graefe also mentioned noticing the venous pulsation of the glaucomatous eye. 2

The History of TonometryErnst Pflger theorised the link between Intra Ocular Pressure (IOP) and Glaucoma and associated changes to the Optic Disc. Schitz developed the impression tonometer in 1897 for the Sclera before updating this to a Corneal plunger device still in the Keeler catalogue in the early 1980s.Weber (1867) and Maklakov (1885) produced first Applanation Tonometers.

Indentation and early applanation devices were invented in the late 19th century. This, in particular, was because of the development of topical local anaesthesia. This allowed contact with the patients eye to be made for prolonged periods, though initial problems with sterility did occur. 3

The History of TonometryIn 1954, Professor Goldmann invented an applanation tonometer with a plexiglass probe that made contact with the eye. Using a coiled spring and lever system, it was very reliable and accurate. As the area applanated was small, ocular scleral rebound and globe rigidity did not affect the IOP results. The Goldmann tonometer is still considered the GOLD STANDARD in tonometry to this day. Should this really be the case?

The Goldmann tonometer is a very reliable device which is also fairly easy to use. The recent issues with new variant CJD have led to the increased use of disposable tonometer prisms. There appears to be no evidence of a drop in accuracy with such disposable prisms. Glaucoma shared care requires that Goldmann tonometry is the method used to monitor patients.4

Von Graefe Tonometer

Schitz Tonometer

Draeger Hand Applanation Tonometer

Goldmann and Perkins Tonometers

The History of TonometryIn 1972, AO introduced the Non Contact Tonometer (NCT). It used a pneumatic air puff to applanate the tonometer. Much easier to use, it allowed for quicker IOP screening on all patients. The accuracy and repeatability were not as good as the Goldmann.In 1986, Keeler introduced the Pulsair. This revolutionised quick IOP screening as it was less noisy or uncomfortable than earlier NCTs.

Pneumotonometers first appeared in the mid 1960s. Generally, they used a continuous flow of gas which applanated the Cornea continuously and the applied pressure altered until the live IOP reading could be taken. This took too long and caused discomfort and drying. Later devices used more rapid measuring systems, primarily with the development of faster and smaller transistors and micro-processors. 7

Keeler Pulsair / Intellipuff NCT

AmericanOptical NCTDiaton tonometer

Since the late 1980s, auto tonometers have developed into highly reliable and versatile products, many versions adding auto refraction and auto keratometry functionality as well. Also, through the lid tonometry was introduced in the late 1990s; the Diaton promised excellent results without discomfort, but the added range of biomechanical properties introduced by the eye lid and tarsal plate simply makes results less reliable and adds more variables to take into account.8

Reichert PT 100

Reichert Tonopen and Tonopen XL

Reichert 870

Here we have examples of three varieties of device. The Tonopen uses a slightly different method of measuring the intra ocular pressure (see later).9

The History of TonometryCurrent devices most commonly found in everyday practice tend to be combined pneumo tonometers and auto refractors / auto keratometers. Most recent devices also analyse corneal pachymetry and in some cases, also evaluate corneal hysteresis or bio-mechanical properties. There is also a contact tonometer which evaluates ocular pulse amplitude for short term IOP fluctuations (Pascal).

Nidek Tonoref III

Oculus Corvis ST

Reichert Corneal Response Analyser

Zeimer Pascal DCT

These more modern devices add more than just standard tonometry (see later).11

Tonometry relevance?Most studies show IOP (intraocular pressure) is the biggest risk factor in developing glaucomaMore precise knowledge of how IOP causes mechanical damage or vascular damage or a combination of both now existsApoptosis of Ganglion cells and the reduction in axonal transport (RNFL) is the proven cause of actual loss of Ganglion cells and their axons in the RNFL; ONH blood perfusion?

Intra ocular pressure was considered a primary diagnostic factor in glaucoma. However, the definition of glaucoma is an optic neuropathy presenting progressive loss of neural tissue leaving the eye at the optic nerve head. There is no mention in this definition of intra ocular pressure. More and more cases of ocular hypertension without associated field loss OR low / normal tension glaucoma without raised intra ocular pressure have now been documented which cast doubt on IOP as the primary cause. Where IOP is a cause, there are two main likely causes of cell death; Direct mechanical damage and indirect ischaemia to neural tissue. This can lead to vascular damage, closing off of axonal transport in the RNFL and the release of glutamate in the vitreous thought to cause excitotoxicity to the ganglion cells. Programmed cell death results. The cause of this relies on two theories; Selective Damage Theory and Reduced Redundancy Theory.12

Tonometry relevance?There are numerous papers which show that reducing IOP in normal tension glaucoma is still effective for many patients.The exact link between IOP and Glaucoma is still not fully understood. Ocular blood flow and even the physical architecture of the Lamina Cribrosa have been shown to have some influence on Glaucoma prevalence.

Tonometry Gold StandardThe current recognised Gold Standard tonometer is the Goldmann Applanation Tonometer (GAT). The GAT is a mechanical device introduced in 1956 and relies on the Imbert Fick law, a 3.06mm diameter probe making contact with the cornea and the assumption that the cornea is 520 microns thick.There are many ways the measured IOP can be incorrect with GAT.

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The Bi Prism of the Goldmann allows the user to know when the probe is lined up correctly and when applanation has been achieved.

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Poor alignment of the Mires is the most common cause of incorrect readings and taking too long.

Too much Fluorescein

Too little FluoresceinMain Errors with Goldmann

Adding too much Fluorescein can make the mires too thick. If this occurs, ask the patient to blink rapidly for a half a minute. You could use the edge of an absorbent tissue in the lower fornix whilst the patient looks up to remove excess Fluorescein, but avoid abrasion or adding lint to the tear film. If the mires are too thin, simply add a little more Fluorescein, either with a strip or adding more of the drop.17

Dial pressure too low, increase pressure on dial so mires just overlap.Dial pressure too high, reduce pressure on dial so mires just overlap.Probe too high, move the Probe. Vice Versa is lower mire is biggest.Main Errors with Goldmann

Other errors occur when the patient has not been settled and their IOP is raised due to them being nervous and apprehensive. Also, remember to take averages due to the pulse and remember diurnal variation too.18

Every GAT should be supplied with a calibration rod. Essentially, this is the same as the weights that are used with the Perkins. There are three settings to calibrate against, 0 mmHg, 20 mmHg and 60 mmHg.Goldmann Calibration

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Insert the calibration bar, with the zero notch in the centre first, as shown here. With the dial set below zero, slowly turn the dial until the probe arm just tilts forwards. The scale should read zero if the GAT is properly calibrated for 0 mmHg. If not, it will need sending for recalibration. Repeat this for 20 and 60 mmHg.

It is estimated only 83% of GATs sold undergo calibration and often this is sporadic and not routine. Regular calibration is essential and a log of each check (the time, date and who did it) should be kept. Every three months is a fair period to repeat this, but some suggest even doing this daily!20

Rebound Tonometry

The iCare TonometerThis is particularly useful in domiciliary optometry or where patients refuse anaesthetic for Goldmann.

Some papers do suggest over estimation of the IOP with the iCare compared to other portable tonometers where as other studies show good correlation with the Goldmann and the iCare.Rebound technology is based on a rebound measuring principle, in which a very light-weight probe is used to make momentary contact with the cornea. In rebound technology, the motion parameters of the probe are recorded during the measurement. Induction based coil system is used for measuring the motion of the probe. Advanced algorithm combined with state of the art software analyses the probe deceleration, contact time and other parameters of the probe while it touches the cornea. The deceleration and other rebound parameters of the probe change as a function of IOP. In simple terms, the higher the IOP, the faster the probe decelerates and the shorter the contact time.21

Rebound Tonometry

The home use version has been shown to be very effective for self checking of IOP and assessment of long term diurnal variation

Electronic Indentation Tonometry

The Tonopen range of tonometers use a combination of applanation and indentation to measure the IOP.

The Tono-Pen involves both applanation and indentation processes. It is a small, handheld, battery-powered device. The tonometer has an applanating surface with a tiny plunger protruding microscopically from the centre. As the tonometer -makes contact with the eye, the plunger gets resistance from the cornea and IOP producing a rising record of force by a strain gauge. At the moment of applanation, the force is shared by the foot plate and the plunger resulting in a momentary small decrease from the steadily increasing force. This is the point of applanation which is read electronically. Multiple readings are averaged. Because the area of applanation is known, the IOP can be calculated. The readings correlate well with Goldmann tonometry within normal IOP ranges23

Tonometry and PachymetryCorneal Centre Thickness (CCT) is assumed to be 520 microns with the Goldmann tonometer. Most other tonometers assume either 545 or 550 microns for the CCT. If the CCT is higher than that assumed by the tonometer, then the device will OVER ESTIMATE the patients IOP.If the CCT is lower than that assumed by the tonometer, then the device will UNDER ESTIMATE the patients IOP.

Measuring pachymetry is clearly important in correctly assessing a patients IOP. In Scotland, this is now a requirement and recently a directive to supply every practice with a pachymeter was put into place. Soon, every practice in Scotland will receive an Accutome pachymeter from Keeler.

REMEMBER post LASIK / LASEK patients.24

Is Pachymetry enough?CCT certainly helps us to calculate a more accurate IOP reading. Remember to consider the patients pulse; take averages. NICE suggests 4 readings per eye with pneumo tonometers.Also, diurnal variation and even longer term variations have been shown to exist. Phasing of IOP readings might be worthwhile.

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What is Corneal Hysteresis?

The corneal hysteresis is the amount of energy that is absorbed by the system.

Therefore there is a difference in the loading and unloading process.

The area between the loading and the unloading curve is the exact measure of corneal hysteresis.A perfect elastic material would have no hysteresis

The Cornea is a Viscoelastic material!

The cornea has an elastic and a viscosic component.The collagen fibres of the stroma provide the highest contribution for the stiffness of the cornea.The collagen fibres are embedded in a gel-like substance (proteoglykanes) that cause friction.

Providing REAL IOP readings

Reichert Ocular Response AnalyserOculus Corvis ST Biomechanical Tonometer

Now that we have examined briefly the potential pitfalls of simple applanation tonometry, the value of measuring pachymetry and the added value of understanding corneal hysteresis, we can consider devices which take all of these things into consideration and put them together. The results from such devices are shown to be far more reliable, repeatable and accurate. However, such devices can even go beyond the realms of just providing real IOP measurements. 28

Oculus Corvis

4300 frames per second Scheimpflug camera image records the corneal reactions from a 60mmHg pneumo applanation

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Oculus CorvisContact Lens ExperimentTAN 40 at 0mm Hg adjusted chamber pressureContact Lens ExperimentTAN 40 at 20mmHg adjusted chamber pressureContact Lens ExperimentTAN 40 at 60mmHg adjusted chamber pressure

Deformation amplitude significantaly decreases when pressure increases

This shows the affect of IOP on the same thickness contact lens in a pressure chamber. Notice the deformation decreases as the IOP increases, but not linearly.30

Deformation amplitude at 20 mmHg significantely decreases with higher stiffness of materialOculus Corvis

Here, we can see the affect where a different thickness of material changes the corneal response with the same IOP. Again, this is not a linear relationship.31

Beyond Tonometry

Conclusion: Corvis ST with best inter- and intra- observer reproducibility compared to NCT (Topcon) and GATAuthor: Jiaxu Hong1, Jianjiang Xu1, Anji Wei1, Sophie X. Deng2, Xinhan Cui1, Xiaobo Yu1,

The Corvis shows the best reproducibility of IOP results when compared to the Gold Standard Goldmann Applanation Tonometer (GAT) or to one of the more accurate Non Contact Tonometers.32

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How big is the influence of corneal thickness on IOP-measurements?

A: up to 2 mmHgB: up to 5 mmHgC: up to 7 mmHgD: >10 mmHg

Answer: Up to 2 mmHgPot. error moderateBeyond TonometryLiu and Roberts, JCRS, January 2005

The thicker the cornea, the higher the measured pressure

The thinner the cornea, the lower the measured pressure

IOP-Error

There is a non linear relationship between the difference in CCT (Corneal Centre Thickness) and how much the IOP is affected. 33

How big is the influence of stiffness on IOP-measure-ments?

A: up to 2 mmHgB: up to 5 mmHgC: up to 7 mmHgD: > 10 mmHg

Antwort: > 10 mmHgPot. Error HugeBeyond Tonometry

IOP-Error

The stiffer the cornea, the greater the measured pressure

The softer the cornea, the lower the measured pressure

More worryingly, corneal stiffness (hysteresis) produces huge potential errors in the IOP measurements of standard tonometers, including GAT with pachymetry. This is why I propose the Corvis become the new Gold Standard in Optometry. 34

Corneal Elasticity and Corneal Thickness

The effect of CCT depends on Youngs modulus of elasticity

Difference = 8.68mmHgDifference = 2.84mmHgLiu and Roberts, JCRS, January 2005

Difference = 8.68mmHg

Older Patient = Stiffer cornea

Higher influence of conneal thickness on IOP

Difference = 2.84mmHg

Young Patient = Weaker cornea

Smaller influence of corneal thickness on IOP

Takes into account age + corneal thicknessIOPcorr.= IOPmeas + constant*age*(540-CCT)Sporl Correction Table

Data provided by Renato Ambrosio, MD, PhD

Normal eye versus Keratonic Eye

We may therefore use this device to identify patients at risk of or suffering from keratoconus or other ectasia (Pellucid marginal degeneration for example).37

greater stability in post-op blue image than in pre-op red imageData provided by Renato Ambrosio, MD, PhD

Red: Keratoconus cornea Blue: same cornea after CXLAssessing Collagen Cross Linking (CXL)

Here we can see the affect of collagen cross linking on the stiffness of the patients cornea. This shows the added value of modern biomechanical tonometers. 38

Hong J, Xu J, Wei A, Deng SX, Cui X, Yu X, Sun X.:A new tonometer--the Corvis ST tonometer: clinical comparison withnoncontact and Goldmann applanation tonometers.Invest Ophthalmol Vis Sci. 2013 54(1):659-665 Reznicek L, Muth D, Kampik A, Neubauer AS, Hirneiss C.:Evaluation of a novel Scheimpflug-based non-contact tonometer in healthysubjects and patients with ocular hypertension and glaucoma.Br J Ophthalmol. 2013 Leung CK, Ye C, Weinreb RN.An ultra-high-speed Scheimpflug camera for evaluation of corneal deformationresponse and its impact on IOP measurement.Invest Ophthalmol Vis Sci. 2013 54(4): 2885-2892 Kling S, Marcos S.Contributing factors to corneal deformation in air puff measurements.Invest Ophthalmol Vis Sci. 2013 54(7):5078-5085 Faria-Correia F, Ramos I, Valbon B, Luz A, Roberts CJ, Ambrsio R Jr.:Scheimpflug-based tomography and biomechanical assessment in pressure inducedstromal keratopathy.J Refract Surg. 2013 29(5):356-3588 Ambrsio R Jr, Valbon BF, Faria-Correia F, Ramos I, Luz A.:Scheimpflug imaging for laser refractive surgery.Curr Opin Ophthalmol. 2013 Jul;24(4):310-20References

College of OptometristsTonometers Peter C. Kronfeld.:The History of GlaucomaGlaucoma Volume 3, Chapter 41 Michael Tsatsos, David BroadwayControversies in the history of glaucoma: is it all a load of Greek?Br J Ophthalmol. 2007 Nov; 91(11): 1561 - 1562 Ivan MarjanovicThe history of detecting glaucomatous changes in the optic dischttp://dx.doi.org/10.5772/52470 Kniestedt C, Nee M, Stamper RLDynamic contour tonometry: a comparative study on human cadaver eyes.Arch Ophthalmol. 2004 Sep: 122(9): 1287-93 Aghaian E, Choe JE, Lin S, Stamper RLCentral corneal thickness of Caucasians, Chinese, Hispanics, Filipinos, African Americans and Japanese in a Glaucoma clinic.Ophthalmology. 2004 Dec; 111(12):2211-2219References

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